Data storage disks are produced using a disk replication process. A master disk is made having a desired surface relief pattern formed therein. The surface relief pattern is typically created using an exposure step (e.g., by laser recording) and a subsequent development step. The master is used to make a stamper, which in turn is used to stamp out replicas in the form of replica disk substrates as part of a disk molding process. As such, the surface relief pattern, information and precision of a single master car be transferred into many inexpensive replica disk substrates.
Conventional mold assemblies typically include a fixed side and a moving side. The stamper may be typically attached to the moving side for replicating a desired surface relief pattern (i.e., lands, grooves and/or pits) into the replica disk substrate. A movable gate cut may be provided for cutting a central opening in the replica disk substrates. The stamper may be secured to the moving side using an inner holder, wherein the inner holder fits over the stamper. Several more tooling parts may be located at the enter of the mold assembly.
During the disk molding process, a resin, typically optical grade polycarbonate, is forced in through a sprue channel into a substrate cavity within the mold assembly to form the replica disk substrate. The surface relief pattern or formatted surface is replicated in the replica disk substrate by the stamper as the cavity is filled. After filling, the gate cut is brought forward to cut a center hole in the replica disk substrate. After the replica disk has sufficiently cooled, the mold assembly is opened and the gate cut and a product eject may be brought forward for ejecting the formatted replica disk substrate off of the stamper. The inner holder may be removable to allow changeout of the stamper.
While the resin is forced into the substrate cavity of the mold assembly by the molding press. Injection pressure overcomes clamp force causing mold to open a small amount (commonly termed "mold blow"), pressure is then increased to the mold assembly to clamp the mold shut, forcing the resin into the microscopic surface relief pattern of the stamper (which contains the reverse image of the desired replica disk surface relief pattern). Thus, the above process is commonly termed "injection compression" or "micro-coining".
The resin is required to be highly viscous so that it may flow into the microscopic detail of the stamper surface relief pattern. The molding process requires the mold assembly to have the capability to "breath" or to open a small amount after injection of the resin into the disk substrate cavity. As the mold assembly "breathes", small openings may be created around the perimeter of the mold assembly, allowing for resin to enter into the openings, causing what is known as mold "flash". Due to the viscous nature of the resin, even very small openings may allow the resin to flash.
Mold flash may cause several problems. Such problems include out of specification parts or contamination from debris generated from the flash area during use of the disk. FIG. 1 is a partial cross-sectional view illustrating a prior art mold. As shown, some optical media molds employ a telescoping shutoff to compensate for mold blow. With a telescoping shutoff, one side of the mold enters into the cavity of the other, where the contacting surfaces or running surfaces are at a very slight angle (indicated at A and B). The telescoping shutoff allows the mold assembly to open a small amount, allowing minimal flash but does require the mold halves to be perfectly aligned to one another.
For single layer optical disk media formatted on one side, mold assemblies have been designed to limit flash such that it occurs only on the non-functional areas of the disk. By limiting the flash to the non-functional areas of the disk, interference problems caused by the flash may be reduced.
As optical disk technology has evolved, optical disks have increased in storage capacity. Higher density disks have resulted in the storage of a greater amount of information (areal density) within the same size of disk surface area. Higher density optical disk formats are required to be manufactured having smaller track pitches. For increased storage capacity, these disks may include formatted surfaces on both sides (i.e., typically termed double-sided disks). Double-sided disks are made by placing two single-sided disks (i.e., two single layer disks formatted on one side) back-to-back. These higher areal density disks have stricter requirements and tighter design criteria, requiring more complex molding processes. As data storage capacity increases, often problems which may result from flash or other mold blow problems are magnified.
For disk formats utilized in flying head applications, as disk capacity increases the design tolerances for the desired surface relief pattern become more critical. The flying heads may be required to pass closer to the disk substrate, requiring tighter disk specifications, including a reduction or elimination of disk surface geometry imperfections, such as mold flash protrusions.